This patent document relates to techniques and devices that generate laser pulses in the mid-infrared (mid-IR) spectral region for optical sensing in various applications.
Light in the mid-infrared (mid-IR) spectral region including optical wavelengths of 2.5-12 μm is resonant with some fundamental transitions of molecular bond vibrations and thus can be used a sensitive probe for studying or identifying molecular structures. In various applications, mid-IR spectroscopy provides useful analytical techniques for identifying and characterizing molecules. Localized vibrations have fundamental transitions from 2.5 to 6 μm, while delocalized modes from 6 to 12 μm are sensitive probes of the overall molecular structure. Combined, the mid-IR spectrum provides a molecular fingerprint. Overtones of the fundamental modes at wavelengths shorter than 2.5 μm are sometimes studied but are often weak and difficult to interpret. Linear spectroscopies generally utilize a broadband, incoherent blackbody source to cover the entire mid-IR range. Nonlinear optical spectroscopic applications require short, high-energy (several μJ) laser pulses, but generating mid-IR pulses with wide spectral coverage and high energies simultaneously is challenging. High-energy pulses for nonlinear mid-IR spectroscopy are typically generated through nonlinear mixing of the outputs of optical parametric amplifiers (OPAs) and span only a few hundred nm. Although considered broadband, these pulses span only a small fraction of the vibrational spectrum. Nonlinear spectroscopic experiments can be performed by scanning the spectral envelope of such pulses over a wider spectral region, but this is a time consuming process and introduces additional technical difficulties, such as correcting for changes in the beam pointing and temporal overlap.